[ntp:questions] Re: Clock accuracy & auto setting : digital television does a crap job of providing time services...

John Ackermann N8UR jra at febo.com
Mon May 1 13:32:14 UTC 2006


Max Power wrote:
>>>LF time services are OK, and are necessary over large transnational
>>>regions -- like Sub Saharan Africa, Australasia and South America ... but
>>>any new LF service needs to be more technologically advanced than WWVB, 
>>>MSF
>>>or DCF77 and its Swiss twin. In these regions 10 LF frequencies need to 
>>>be
>>>allocated, but the signal to be transmitted needs to be more modern than
>>>WWVB or DCF77 -- maybe using some form of low complexity PSK or low
>>>complexity QAM and 240 hz to 480 hz of bandwidth. The signal must be
>>>futureproofed -- as above.

One thing to remember is that LF stations were originally designed, and 
still serve, another important purpose: to provide precise *frequency* 
measurement capability.  The original intention of WWVB was to provide a 
stable carrier frequency in a spectrum that has minimal ionospheric and 
propagation disturbances.  The original WWVB receivers were 
phase-tracking units that allowed a local frequency standard to be 
directly compared to the USFS.

As in timekeeping, GPS has become the method of choice for frequency 
calibration, but there are still people using WWVB (and presumably other 
standard frequency stations) for their original purpose.  If nothing 
else, it provides direct traceability to NBS without needing to adjust 
for NIST<-->USNO offsets.

One of the advantages of the current modulation scheme is that it 
doesn't mess with the carrier phase.  If more complex modulation schemes 
reduce or destroy the ability to phase track the carrier, one of the 
primary purposes of the LF stations will be lost.

John



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